IN 1990, the discovery of nitrogenation effect on relevant

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1 IEEE TRANSACTIONS ON MAGNETICS, VOL. 51, NO. 11, NOVEMBER Research and Development of Interstitial Compounds Yingchang Yang 1, Jinbo Yang 1,2,3, Jingzhi Han 1, Changsheng Wang 1, Shunquan Liu 1, and Honglin Du 1 1 School of Physics, Peking University, Beijing , China 2 State Key Laboratory for Mesoscopic, Peking University, Beijing , China 3 Collaborative Innovation Center of Quantum Matter, Beijing , China The interstitial rare earth iron compounds exhibiting a variety of magnetocrystalline anisotropies, large saturation magnetization, and high Curie temperature provide the basis for high-performance magnetic materials. In this paper, a manufacturing process for mass production of anisotropic magnetic powders has been successfully developed for Nd(Fe, M) 12 N (NdFeN) and Sm 2 Fe 17 N 3 (SmFeN) compounds with uniaxial anisotropy. The magnetic powders of Nd(Fe, M) 12 N have maximum energy products [(BH) max ] around 22 MGOe. The calendar magnets of NdFeN show the rolling anisotropy and have a (BH) max up to 5.9 MGOe with excellent mechanical properties and good anticorrosion ability. The (BH) max of SmFeN magnetic powders is up to 41 MGOe, which is favorable for fabricating high-performance injection magnets and extrusion magnets. On the other hand, it was found that cone or planar anisotropic R 2 Fe 17 N x (R = Ce, Pr, Nd) and paraffin composites can improve the Snoek limit and exhibit a high microwave reflection loss of up to 35 db at about 14 GHz, presenting an advantage for application as a high-performance thin-layer microwave absorber. These results open up broad prospects for technological applications using interstitial modified compounds. Index Terms Interstitial atom effect, microwave absorbing materials, NdFeN, SmFeN. I. INTRODUCTION IN 1990, the discovery of nitrogenation effect on relevant rare earth iron intermetallics has attracted great interest in the study of rare earth nitrides [1] [3]. The nitrogenation effect is typically demonstrated in R 2 Fe 17 N 3 (2:17, R = rare earth element) and R(Fe, M) 12 N (1:12, R = rare earth element, and M = Mo, Ti, and V) rare earth iron compounds. After nitrogenation, the intrinsic magnetic properties of the 2:17 and 1:12 nitrides are marvelously improved [4], [5], making their intrinsic magnetic properties comparable with those of Nd 2 Fe 14 B, or even better in terms of Curie temperature and the anticorrosion ability. How is the nonmagnetic nitrogen atom able to improve dramatically the magnetic properties of a rare earth iron alloy? Theoretically, it is revealed that the physical origin results from interstitial nitrogen atom effect [6], [7]. Accordingly, in 1990, interstitial compounds, as a new member, appeared in the field of magnetic materials. Since then, a quarter of a century has past; much effort has been devoted worldwide to bring interstitial compounds into commercial applications [8] [16]. Due to intensive fundamental and technical studies dealing with the interstitial compounds, commercialization of the new materials with diverse applications has begun. Since the interstitial compounds are developed on the basis of the interstitial nitrogen (carbon) atom effect, it is necessary to review briefly the interstitial nitrogen atom effect as the first part in this paper. It is well known that appropriate intrinsic magnetic properties provide only the possibility for preparing magnets with high performance. Hard magnetic properties, such as Manuscript received March 20, 2015; revised May 19, 2015; accepted June 1, Date of publication June 5, 2015; date of current version October 22, Corresponding author: Y. Yang ( fuyang@pku.edu.cn). Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /TMAG coercive force, remanence, and maximum energy products are extrinsic properties which depend technically on the manufacturing process, and theoretically on the magnetic domain structure and the demagnetization mechanism. In order to transform the possibility into a reality, we have carried out a systematic study of the magnetization process of the interstitial compounds. As a result, a manufacturing process for mass production of high-performance anisotropic magnetic powders Nd(Fe, M) 12 NandSm 2 Fe 17 N 3 has been successfully developed using updated powder metallurgy techniques. The manufacturing process will be presented in the second part of this paper. Benefiting from the interstitial nitrogen atom effect, a variety of magnetocrystalline anisotropy properties of the interstitial compounds were observed. The magnetocrystalline anisotropy of the interstitial compounds covers not only uniaxial anisotropy, but also cone and planar anisotropies. Due to high Curie temperature and large saturation magnetization compared with those of ferrites, a material with such properties can have a higher Snoek limit, initial permeability, and working frequency. Finally, we will report that the interstitial compounds can be used as promising candidates as microwave absorbers in the high-frequency region. II. EXPERIMENTAL DETAILS The samples were prepared by vacuum induction melting and strip casting 99.5% pure materials in a purified argon atmosphere. Nitrides were prepared by passing purified nitrogen gas at atmospheric pressure over finely ground powder sample at K for 2 10 h and then rapidly cooling to room temperature. Anisotropic magnetic powders were prepared by a milling process. X-ray diffraction (XRD) and neutron diffraction were used to identify the structures and phases of the samples. Magnetic measurements were performed using a vibrating sample magnetometer and a SQUID magnetometer. Magnetic force and Kerr microscopy was used to investigate the domain structure. In order to measure the microwave absorption properties, the ball-milled IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See for more information.

2 IEEE TRANSACTIONS ON MAGNETICS, VOL. 51, NO. 11, NOVEMBER 2015 TABLE I LATTICE PARAMETERS a AND c, UNIT CELL VOLUME V,RELATIVE CHANGE IN THE UNIT CELL VOLUME UPON NITROGENATIONδV/V, CURIE TEMPERATURE T c,saturation MAGNETIZATION M s, AND ANISOTROPY FIELDS μ 0 H a OF RFe 10.5 V 1.5 AND RFe 10.5 V 1.5 Nx [5] Fig. 1. Crystallographic structures of (a) Th 2 Zn 17 -type rhombohedral structure R 2 Fe 17 N 3 and (b) ThMn 12 -type structure R(Fe, M) 12 N (M = Cr, Ti, V, Mn, Mo...). RFeN particles were dispersed into paraffin and then the RFeN/paraffin composite was compressed into a toroidal shape with an inner diameter of 3.04 mm and an outer diameter of 7.00 mm. The absorption properties of the prepared samples were measured using a vector network analyzer. III. RESULTS AND DISCUSSION A. Interstitial Atom Effect Fig. 1 shows the crystallographic structures of typical interstitial 2:17 and 1:12 compounds. Since the size of the nitrogen atom is smaller than that of rare earth and iron atoms, the neutron diffraction studies demonstrate that all of these compounds crystallize in the same structure as their parent alloys, and N atoms occupy the interstitial sites leading to an expansion of the unit cell [11], [12]. The interstitial N atoms have a sensitive effect on raising Curie temperature, increasing saturation magnetization, and enhancing magnetocrystalline anisotropy. The magnetic properties of some R(Fe, M) 12 and their nitrides are listed in Table I [5]. Since the crystal structure of the 1:12 nitrides is much simpler than that of the 2:17 structure, the nitrogenation effect is more completely presented with the 1:12 nitrides; we thus take the 1:12 nitrides as an example to expound the physical origin of the nitrogenation effect. The effects are summarized in three respects. 1) Enhancement of Curie Temperature T c : The average increase in the Curie temperature (T c ) is about 200 K upon nitrogenation for 1:12 compounds. The investigation into the variations of T c with the RFe 10.5 Mo 1.5 and RFe 10.5 V 1.5 series before and after nitrogenation indicates that the variations obey the (g J 1) 2 J(J + 1) law, and the Fe Fe exchange interaction is enhanced and the R Fe interaction is reduced after nitrogenation. It is clear that the increase of T c is due to the enhancement of Fe Fe exchange interaction after nitrogenation. 2) Increase of Saturation Magnetization: Compared with the parent alloys, it is evident that a large increase in the saturation magnetization is achieved upon nitrogenation. The magnetic moment of Fe deduced from hyperfine fields obtained from Mossbauer data and magnetic measurements with yttrium compounds indicates that a large increase in the Fe moment is achieved upon nitrogenation. Accordingly, the increase in the saturation magnetization is related to a modification of the 3d electron band. The band calculation results indicate that the interstitial nitrogen atoms may sensitively and favorably modify the 3d electronic structure associated with Fe in the 1:12 nitrides [6], [7]. The volume expansion and the electronic transformation induced by interstitial nitrogen atoms are responsible for the increase of Fe moment and the enhancement of Curie temperature. 3) Changes in the Magnetocrystalline Anisotropy: Before nitrogenation, Sm is the only rare earth atom whose easy magnetization direction (EMD) is the c-axis in the 1:12 compounds, while the EMDs of the other rare earth atoms lie in the basal plane. After nitrogenation, the EMD of Sm changes from the c-axis to the basal plane, and EMDs of Pr, Nd, Gd, Tb, Dy, and Ho compounds change from basal plane to the c-axis [5]. Because the magnetocrystalline anisotropy is an important characteristic of the nitrides closely related to their potential applications, it is useful to discuss it in detail. The magnetocrystalline anisotropy of the 1:12 nitrides can be classified as follows. 1) The c-axis is the EMD from 0 K to Curie temperature when R represents Pr, Nd, Tb, Dy, and Ho. 2) The EMD lies in the basal plane for R = Sm.

3 YANG et al.: RESEARCH AND DEVELOPMENT OF INTERSTITIAL COMPOUNDS TABLE II MAGNETIC PROPERTIES OF SOME NITRIDES [4], [5] Fig. 2. Dependence of coercivity and remanence on the particle size of anisotropic (a) NdFeN and (b) SmFeN powders. 3) A spin reorientation occurs for R = Er. 4) The uniaxial anisotropy is reduced when R = La, Ce, Gd, and Y, which indicates the magnetocrystalline anisotropy behavior of the Fe sublattice in the 1:12 nitrides. The interstitial nitrogen atoms have a strong effect on the crystal field interaction of the 4f electrons associated with rare earth ions. In the 1:12 nitrides the nitrogen atoms occupy the 2b sites which possess the same I4/mmm point symmetry as the rare earth sites. However, the electronic charge of nitrogen ions is opposite to that of rare earth ions. The calculation results show that the contribution from neighboring nitrogen ions to the second-order crystal field coefficient (A 20 ) is positive and large while neighboring rare earth ions contribute negatively to the A 20. Thus, the second-order crystal field coefficient (A 20 ) of the 1:12 compounds changes in sign from negative to positive after nitrogenation (data not shown). The second-order crystal field coefficient A 20 is usually dominant in determining the EMD. Accordingly, for the 1:12 nitrides, the EMDs of the rare earth ions with a negative second-order Steven s coefficient α J, such as Pr, Nd, Dy, Tb, and Ho, are along the c-axis, whereas those of the other rare earth ions that possess a positive α J such as Sm and Er, lie in the plane. The competition of anisotropy between the Er and Fe sublattices results in a spin reorientation in ErTiFe 11 N. It should be pointed out that the interstitial atom effect is not limited to the nitrogen atoms. Carbon and hydrogen atoms exhibit an effect similar to that of nitrogen atoms in the rare earth iron compounds [14] [16]. The interstitial atom effect can change not only the magnetic properties of 1:12, 2:17, and R 3 Fe 29 (3:29), but also those of Nd 2 Fe 14 B (nitrogenation can increase its Curie temperature by about 60 K). These provide versatile tools to modify the magnetic properties of rare earth alloys according to their application requirements. B. Manufacturing Process for Mass-Production of Anisotropic Magnetic Powders With High Performance Based on the Interstitial Compounds Due to the strong uniaxial magnetocrystalline anisotropy, large saturation magnetization, and high Curie temperature induced by interstitial nitrogen atom effect, SmFeN, NdFeN, and Pr(Fe, M) 12 N(PrFeN) present excellent intrinsic magnetic properties (see Table II). The intrinsic magnetic properties of these nitrides provide the essential requirements for developing permanent magnets with high coercive force, large remanence, and high maximum energy products. We have developed a process to manufacture fine monocrystal magnetic powders of NdFeN and SmFeN using the powder metallurgy technique. The fine monocrystal powders were obtained by the surfactant-assisted milling technique [13]. However, how to achieve a desirable coercivity with monocrystals is the key point in this process. It is worth noting that in this process each particle can be a single crystal, but not in a single domain state. During the demagnetization process, both domain nucleation and domain wall motion should be considered. In fact, NdFeN or SmFeN fine powders presents a strong dependence of i H c on the applied magnetic field H [17], [18] as well as the particle size (see Fig. 2). These facts suggest that the demagnetization process is associated with nucleation of reversal domain and de-pinning of domain boundaries. Therefore, enhancing the strength of the nucleation field and achieving a high coercivity with an ideal squareness for monocrystal materials is the key technique dealing with this process. A three-step production process including strip casting (master alloys), gas solid reaction (nitrogenation), and powder refinement (pulverization) was established to prepare nitrides with high coercivity and remanence. As the starting point, NdFe 10.5 Mo 1.5 or Sm 2 Fe 17 alloys are prepared by a strip-casting method. Then, it is followed by a gas solid phase reaction to form the NdFe 10.5 Mo 1.5 NorSm 2 Fe 17 N 3 nitrides. Finally, the nitrides are further pulverized to fine monocrystal particles by the ball milling technique. Thus, all the particles are aligned when a magnetic field is applied. In order to enhance the nucleation field strength, it is required to minimize the defects during each step of the production process. Therefore, the products have to achieve the following requirements. 1) NdFe 10.5 Mo 1.5 and Sm 2 Fe 17 Master Alloys Are Required to Be a Single-Phase Material: It is well known that in the melt casting process, the rare earth iron alloys, such as Sm 2 Fe 17 alloys, melt peritectically and the conventional induction melt-casting method cannot attain rapid cooling, and thus, precipitation of both α-fe and rich rare earth phases is not avoidable. For this reason, the strip-casting technique is used to produce the single-phase NdFe 10.5 Mo 1.5 and Sm 2 Fe 17 strips as shown in Figs. 3 and 4. Furthermore, the thermomagnetic curves of NdFe 10.5 Mo 1.5 and Sm 2 Fe 17 prove that no α-fe is observed.

4 IEEE TRANSACTIONS ON MAGNETICS, VOL. 51, NO. 11, NOVEMBER 2015 Fig. 5. Room-temperature hysteresis loops of the obtained (a) NdFe 10.5 Mo 1.5 Nand(b)Sm 2 Fe 17 N 3 powders. Fig. 3. XRD patterns of (a) nonaligned NdFe 10.5 Mo 1.5, (b) nonaligned NdFe 10.5 Mo 1.5 N, and (c) aligned NdFe 10.5 Mo 1.5 N powder sample. Fig. 6. Demagnetization curve of the NdFeN calendar magnets. Fig. 4. XRD patterns of (a) nonaligned Sm 2 Fe 17, (b) nonaligned Sm 2 Fe 17 N 3, and (c) aligned Sm 2 Fe 17 N 3 powder sample. 2) Complete Gas Solid Reaction in the Nitrogen Atmosphere Is Indispensable: High anisotropy field H a, large saturation magnetization M s, and high Curie temperature T c are a prerequisite for creating large coercive force i H c, remanence B r, and maximum energy products (BH) max. Thus, a complete gas solid reaction in the nitrogen atmosphere is absolutely necessary. From the XRD patterns of NdFe 10.5 Mo 1.5 N and Sm 2 Fe 17 N 3, as illustrated in Figs. 3(b) and 4(b), α-fe is negligible. Compared with the XRD patterns of their master alloys in Figs. 3(a) and 4(a), all the diffraction lines shift to a lower angle due to the volume expansion after nitrogenation, implying that nitrogen atoms enter their interstitial sites, which guarantees the formation of single phase nitrides. Figs. 3(c) and 4(c) show that the c-axis becomes the EMDs of NdFe 10.5 Mo 1.5 N and Sm 2 Fe 17 N 3, by means of a complete gas solid reaction in the nitrogen atmosphere. 3) Each Particle of the Nitrides Is in a Pseudosingle Crystal State: The performance of the powders is sensitively related to their microstructure, especially their particle size. Fig. 2(a) and (b) shows respectively the dependence of coercive force and remanence of NdFe 10.5 Mo 1.5 N and Sm 2 Fe 17 N 3 powders on their particle size. The nitrides are further pulverized to fine mono-crystal powders with narrow size distribution. The room-temperature hysteresis loops of the obtained NdFe 10.5 Mo 1.5 N and Sm 2 Fe 17 N 3 powders are illustrated in Fig. 5(a) and (b). The NdFeN powders have a moderate (BH) max around 20 MGOe. However, they show a significant advantage for preparing flexible magnets. During the calendar process, the powders were aligned without applied magnetic field. NdFeN is the first rare-earth-based nitride which was found to present a rolling anisotropy in the calendar magnet. The (BH) max of NdFeN calendar magnets is around 5.9 MGOe (see Fig. 6). Especially, NdFeN powders have strong anticorrosion ability. No aging effect is observed with NdFeN powders for five years. The maximum energy products of SmFeN magnetic powders is up to 41 MGOe, favorable for preparing high-performance injection or extrusion magnets. Based on the experimental data of saturation magnetization shown in Table II, the theoretical value of (BH) max for Sm 2 Fe 17 N 3 is 60.0 MGOe at room temperature and 69.4 MGOe at 2 K. Thus, further improvement of the maximum energy products of the nitrides is possible. C. Microwave-Absorbing Materials For many years, research on rare earth intermetallic compounds focuses on finding strong uniaxial anisotropy materials for permanent magnets. Little attention has been paid to materials with complex cone or planar anisotropy. Interstitial compounds with cone or planar anisotropy could become promising candidates as microwave-absorbing materials due to high saturation magnetization and high ratio of the c-axial magnetocrystalline anisotropy field to the basal plane anisotropy field. Metallic magnetic materials and ferrites with the magnetic loss mechanism have been widely studied as microwaveabsorbing materials. Metallic magnetic materials normally show high saturation magnetization (M s ), but low resistivity,

5 YANG et al.: RESEARCH AND DEVELOPMENT OF INTERSTITIAL COMPOUNDS Fig. 7. Frequency dependence of the RL for (a) Ce 2 Fe 17 N X /paraffin composite and (b) Nd 1.25 Sm 0.75 Fe 17 N X /paraffin composite. which could result in high eddy current loss in the gigahertzfrequency range. In contrast, ferrites reveal a much higher permeability μ and resistivity which show low eddy current loss in the same frequency range. However, ferrites exhibit relatively low M s. The microwave absorption performance of the materials is largely determined by the saturation magnetization, natural resonance frequency ( f r ), permeability, and permittivity (ε). Usually, microwave-absorbing materials are composites in the form of magnetic particle-polymer films, and thus, the metallic magnetic materials can be used at high frequency. It is necessary to use the particles smaller than the skin depth for reducing eddy current loss. As we all know, for the bulk materials with cubic structure, it is impossible to increase both the f r and the initial permeability (μ i ) at the same time, due to the Snoek limit [19] f r (μ i 1) = 1 3π γ M s (1) where γ is 2.8 MHz/Oe, named gyromagnetic ratio. Obviously, the right part of the expression is constant. However, as for the materials with planar anisotropy and low structure symmetry, the Snoek limit can be described by f r (μ i 1) = 1 4π γ M H aθ s (2) H aφ where H aθ is the out-of-plane anisotropy field and H aφ is the in-plane anisotropy field. Normally, H aθ is always much larger than H aφ. Compared with (1), the value of the right part in expression (2) could be much larger. Accordingly, the Snoek limit could be increased and a microwave-absorbing material with high working frequency and permeability could be achieved [20]. Therefore, it will be interesting to find materials with high M s and high ratio of H aθ to H aφ. In view of the fact that interstitial compounds show high saturation magnetization, Curie temperature, and the variable magnetocrystalline anisotropy field, they can be good candidates as microwave-absorbing materials. We have studied microwave absorbing properties of the rare earth ironnitrides (RFeN) and prepared R 2 Fe 17 N X (R = Ce, Pr, Nd, and mixture of Sm and Nd) magnetic powders. For example, Ce 2 Fe 17 N X shows a planar anisotropy, with higher Curie temperature (718 K) and higher saturation magnetization (160 emu/g) compared with Ce 2 Fe 17. Fig. 7 shows the typical relationship between reflection loss (RL) and frequency for Ce 2 Fe 17 N X /paraffin and (Sm, Nd) 2 Fe 17 N X /paraffin composites with various thicknesses. Considering that RL = 20 db is comparable with 99% of microwave absorption, the composites show excellent microwave absorption effectiveness. The RL values less than 20 db were obtained in the frequency range GHz with an absorber thickness of mm for the Ce 2 Fe 17 N X /paraffin composite. For the (Sm, Nd) 2 Fe 17 N X /paraffin composite, the RL values less than 20 db were obtained in GHz with an absorber thickness of mm. In particular, the minimum RL value of 35 db was obtained at 14 GHz with a thickness of 1.9 mm. Compared with common absorbing ferrite materials which have been studied in [21] [23], the working frequency and the RL values of these composites are higher. The Ce 2 Fe 17 N X /paraffin composite shows a permeability of μ = 3.2 at a low frequency, which is better than that of M-type ferrite composites [23]. The R 2 Fe 17 N X composite shows good microwave absorption properties due to its high saturation magnetization, high ratio of the c-axis anisotropy field to the basal plane anisotropy field, and suitable particle size. 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